This invention relates to quinacridone pigments. More particularly this invention relates to a process for producing the gamma crystalline phase of quinacridone pigment (hereinafter termed gamma quinacridone).
Linear quinacridone is represented by the structure ##SPC1##
And, as is disclosed in U.S. Pat. No. 2,844,581, exists in a number of crystalline phases, including the alpha phase, the beta phase and the gamma phase.
Recently, gamma quinacridone pigment has found a ready market in the automotive and architectural industries in view of the fact that this pigment produces a deep red metallic finish in the quinacridone color region. Heretofore, pigment size gamma quinacridone has been commercially produced by milling quinacridone after it is produced by oxidation of dihydroquinacridone. Note U.S. Pat. No. 2,844,581. Other patents in this same general area are U.S. Pat. Nos. 3,160,510; 3,257,405 and 3,342,823. The milling of quinacridone to produce the gamma quinacridone pigment is, of course, an additional step in the production of the pigment and requires additional energy input, time, equipment, etc.
It is an object of this invention to produce gamma quinacridone. It is a further object of this invention to produce gamma quinacridone directly from the ring closure of dianilinoterephthalic acid. These and other objects will become apparent from the description which follows.